We propose to study the nature of suppressor cells that appear in the organism at the early stage of tumor development and that actively inhibit the host's immune response. The studies will be carried out in mice with several types of tumors including leukemias and solid tumors. The aim is to elucidate the initial interaction between tumor cells and host cells and subsequent immunoregulatory circuits leading to active immunosuppression. The cell surface structures and the cell-free products involved in these interactions as well as the immunological specificity of the suppressor cells will be studied.